Detachment anticlines formed above a stratigraphically fixed detachmenthorizon and without thinning in the syncline will, in general, requirelayer-parallel shortening. A kinematic model for detachment folds ispresented that has planar limbs and is area-balanced but requiresbed-length changes that can be expressed as layer-parallel strain. Thefold grows primarily by limb rotation, not by limb lengthening atconstant dip. The kinematic evolution of the geometry and strain isexamined. Fold geometry is well predicted and the strain estimated inapplications of the model to two map-scale detachment folds; a trianglezone from the front of the Wyoming thrust belt and a duplex-coredanticline from the Appalachian Plateau.